Rotary joints are employed to introduce a heat exchanging fluid medium into a heat exchanger drum, or remove the medium from the drum. Such joints are widely used in the paper and fabric industries for drying a web rapidly moving over the drum surface, and the most common heat exchanging medium utilized is steam. If the drum is used for cooling purposes the rotary joint may convey water.
Typical rotary joints include a housing rotatably mounted upon a nipple concentric to the drum axis of rotation and rotating therewith. Annular seals interposed between the joint housing and the nipple produce a fluid-tight sealing, and as the seals are mounted within the rotary joint housing the seals are exposed to the internal pressure within.
The pressure within the rotary joint tends to bias the seals against the sealing areas and such forces cause the seal pressures to be excessive accelerating seal wear and increasing the seal friction which elevates the seal temperature further increasing wear.
Several approaches have been utilized with rotary joint construction to minimize excessive seal wear due to seal pressure produced by the pressurized medium being conveyed. Internal selfcompensating joint constructions use designs wherein the seal faces exposed to the internal pressures substantially counteract each other to reduce seal wear. Also, external self-compensating apparatus is used with rotary joints to impose an axial force on the joint housing itself to reduce seal wear and typical examples of this type of external compensation are shown in U.S. Pat. Nos. 1,896,062; 2,700,558; 3,098,665 and 3,874,707.
In known externally compensated rotary joint systems an expansible chamber motor is axially fixed with respect to the rotary joint housing which is axially displaceable, and the motor is concentrically located with respect to the axis of rotation of the drum nipple and rotary joint housing. The interior of the joint housing directly communicates with the interior of the expansible chamber motor which includes a piston, or piston structure, engaging the housing. As the compensating force exerted on the housing by the expansible chamber motor equals the pressure within the housing times the piston area of the motor the degree of compensation achieved is always proportional to the pressure within the rotary joint housing, and the degree of compensation achieved is a fixed percentage of the housing pressure as determined by the area of the piston within the compensating expansible chamber motor. Thus, due to the "fixed" ratio of compensation it is not possible to vary the compensation once the equipment is installed and the degree of compensation can only be varied by making major mechanical changes, such as by utilizing different sizes of pistons.
Expansible chamber motors of the external types used for compensating rotary joints either use pistons having O-rings for sealing purposes, or the pistons are of a metal diaphragm type. Both piston arrangements create problems. Pistons utilizing O-rings seals limit the use of the compensator with respect to high steam pressures due to the temperature limitations of the material of the O-ring as O-ring materials quickly deteriorate at elevated steam temperatures. Diaphragm type piston structures have limited axial travel due to the inherent nature of construction of the metal diaphragm, and as bearing wear occurs and increased piston movement is required for compensation adjustments must be made in view of the limitations of metal diaphragm pistons.
The concepts of the invention overcome the aforedescribed limitations of known external rotary joint compensators.
It is an object of the invention to provide an external load bearing compensator for rotary joints wherein previous problems encountered with the piston structure of prior compensating expansible chamber motors are eliminated.
Another object of the invention is to provide an external self-compensator for rotary joints utilizing an expansible chamber motor wherein the pressurized medium within the expansible chamber motor is a separate medium from that within the rotary joint, and the expansible pressurized medium is not at an elevated temperature and is non-corrosive.
An additional object of the invention is to provide an external self-compensator for rotary joints wherein the degree of compensation can be easily and accurately varied for each rotary joint without requiring structural modifications.
Yet a further object of the invention is to provide an external self-compensator for rotary joints wherein high pressure steam is within the joint housing and compressed air is employed to produce the compensating forces upon the rotary joint.
In the practice of the invention the basic structural arrangements previously utilized with conventional externally compensated joints are employed. For instance, the rotary joint includes axially displaceable bearings to seal the housing interior with respect to the rotary drum mounted nipple upon which the housing is mounted. The housing is mounted for limited axial displacement. An expansible chamber motor includes piston structure coaxial with the joint housing axis of rotation and is axially fixed and engages the housing whereby displacement of the piston axially displaces the rotary joint housing.
However, rather than the chamber of the expansible chamber motor directly communicating with the interior of the rotary joint housing as is the usual practice, the expansible chamber motor receives compressed air having a controlled pressure. The compressed air received by the expansible chamber motor is controlled by sensing and regulating means which senses the pressure within the rotary joint by sensing the pressure within the header supplying steam or other pressuring medium to the rotary joint. Accordingly, it will be appreciated that the pressurized medium being supplied to the rotary joint differs from that being supplied to the compensating expansible chamber motor.
A transmitter is used to sense the pressure within the steam header, or within the rotary joint housing. This transmitter controls multiplying and amplification apparatus which regulates the pressure of compressed air supplied to the compensating expansible chamber motor. The multiplying and amplifying apparatus may be very easily regulated to accurately control the pressure within the compensating expansible chamber motor, and in this manner the compensation of each rotary joint may be "customized", i.e. the axial force imposed upon each rotary joint may be very accurately regulated to compensate for the individual characteristics of that particular rotary joint.
As the pressurized medium being supplied to the compensating expansible chamber motor is clean, cool compressed air the expansible chamber piston may include an elastomeric diaphragm to provide one hundred percent sealing between the piston and the cylinder wall of the expansible chamber motor, and the diaphragm may be so constructed as to provide sufficient axial piston movement without necessitating adjustment throughout the entire compensating range as the seals wear. The use of compressed air eliminates the previously experienced corrosion and deterioration of seal material, and piston structure utilizing a high temperature elastomer and fabric will provide effective sealing for a long period of time while permitting extensive piston movement during compensation.